1. Field of the Invention
This invention involves a system and a method for generating images of an interrogation region based on the echoes from ultrasonic signals transmitted into the region as a planar wavefield.
2. Description of the Related Art
In the area of diagnostic ultrasonic imaging, there is a never-ending effort to increase the resolution, speed, fidelity and affordability of the imaging system. It is in general not possible, however, to improve any one of these factors without worsening at least one of the others. Increases in the resolution and fidelity of the image, for example, often require expensive increases in the number of elements in the transducer array and time-consuming computational complexity in transmission and reception beamforming and in scan conversion.
Moreover, in certain instances, it is impossible to trade any of the other factors off to increase the frame-rate (one aspect of speed) of conventional systems. Assume, for example, that one wants to make the ultrasonic images appear more life-like and "real-time," so that one views the imaged interrogation region as a continuously changing display. One would at first possibly think simply to transmit more pulses into the interrogation region (usually, a portion of a patient's body) with shorter separation in time, that is, to send more pulses per second. The interrogation, however, is a physical system, and no matter how fast the system's processing circuitry is, it cannot reduce the time it takes for sound waves to propagate round-trip between the transducer and the portion of the patient's body being imaged.
For example, at a typical propagation velocity of around 1540 m/s, the round-trip time for an ultrasound pulse to and from a depth of 15 cm would be roughly 200.mu.s. For imaging a volume, a typical frame is made up of on the order of 100 beams, while the column may be made up of on the order of 100 frames. The acquisition time for such a volume would thus take some 2 seconds, which is too long for most cardiac applications.
To reduce the possibility of erroneous signal interpretation due to overlapping (where a second signal is transmitted before the first has returned), one might use separate transducer elements to transmit and receive. This would mean, however, a reduction in achievable resolution for a transducer of any given physical size because not all elements would be used in the transmit and receive stages.
These necessary trade-offs are even more troublesome when it comes to three-dimensional imaging, since the computational burden and memory requirements increase exponentially with the dimension of the image. Using conventional technology, for example, a highly-complicated transmission (and reception) beamforming system may be needed to generate and analyze on the order of a hundred separate beams in order to image a three-dimensional region of a typical size. Beam generation is made even more complicated by the need to properly time, phase-shift, and weight the signals applied to each of the many transducer elements in order to focus the beam in the many regions used to make up the image. What is needed is therefore a way to generate ultrasonic images that is fast enough to create even three-dimensional images without undue hardware complexity and whose speed is less limited by the physical system being imaged.